The present invention relates to an electric photograph system for performing analog printing and more particularly, to a technique for use in a laser printer of a light intensity modulation type wherein a light output power versus current characteristic of a light beam emitted from a semiconductor laser at a focusing position onto a photo-conductor drum is made to be linear so that the quantity of light can be changed to one of a plurality of power levels accurately and therefore printing dots arithmetically changing in diameter or size are generated, thus realizing high-resolution multiple density printing.
As a printer for analog printing, there is known an electric photograph system (electric photograph recording system). One of the electric photograph systems is a laser printer (laser beam printer) which has two types, that is, pulse width modulation and light intensity modulation.
The laser printer of the light intensity modulation type controls a light output power of a semiconductor laser element according to image information to change the spot size of a light beam focused on a surface of a photo-conductor drum (photo conductor) and to control the size of a printing dot for multiple density printing.
An image quality adjusting device for the laser printer is disclosed, for example, in JP-A-3-269456. This literature discloses a technique for solving the problem of non-linear light output power versus characteristics at the time of multiple density printing by increasing the sensitivity level of a photo sensitive material to use only a good linear area thereof.
Also disclosed in xe2x80x9cSID 9 DIGESTxe2x80x9d, pp. 278-299 is a technique for modifying a dot size or printing position. Further disclosed in IEEE Journal of Quantum Electronics, Vol. QE-21, No. 8, Aug.1985, pp. 1264-1270 is mode-hopping noise in a semiconductor laser.
The inventors of the present application suggest, for the purpose of improving the performance of multiple density printing in a laser printer, a technique for controlling the small spot size of a laser beam by setting a light intensity distribution of the laser spot to be triangular (refer to JP-A-9-74251). In this literature, in order to set the light intensity distribution of the laser spot to be triangular, a non current injected area having a length of about 70 xcexcm is provided at halfway (at a position about 30 xcexcm away from its end) of a stripe-shaped waveguide so that the phase difference between fundamental and higher modes is xcfx80/2 at a laser facet.
A technique for providing a non current injected window area at both facets for the purpose of preventing destruction of the facets from which a light beam of a semiconductor laser is emitted, is disclosed, for example, in JP-A-62-65391 and JP-A-62-179193. However, these techniques fail to disclose an application example of improving the linearity of a light output power versus current characteristic of a semiconductor laser.
The electric photograph system (electric photograph recording system) has a function of scanningly directing the spot of a light beam emitted from a semiconductor laser element according to image information onto a surface of uniformly charged photo conductor (such as a photo-conductor drum) for exposure thereof, emitting electric charges therefrom in such a manner that the potential of the surface of the photo-conductor drum is reduced to zero to thereby form an electric potential image, and changing the quantity of a light beam to a plurality of levels at the time of the scanning exposure to change the size of the electric potential image for multiple density printing.
In the prior art laser printer, multiple density printing is carried out by controlling the light output power of a semiconductor laser as a light source to a plurality of intensities to control the size of printing dot.
Explanation will now be made as to an exposing optical system of a general laser printer (laser beam printer), by referring to FIG. 21. In the printing of the laser printer, as shown in FIG. 21, a laser beam 2 emitted from a semiconductor laser 1 is made parallel or collimated by a collimate lens 3, and the collimated beam is once focused on a polygon mirror 5 by a cylindrical lens 4. The laser beam 2 reflected by the polygon mirror 5 is focused through a non-spherical lens system 6 on a drum coated with a photo conductor 7, that is, on a photo-conductor drum 8, so that the photo-conductor drum 8 is scanned with the beam at a constant speed along the axial direction of the drum. The surface of the photo-conductor drum 8 is previously charged uniformly, so that, when the drum is scanned with the laser beam, electric charges on the surface are discharged therefrom and thus the surface potential of the photo-conductor drum 8 is reduced to zero.
When toner particles are electrically adsorbed on an electric potential image thus formed, a toner image is formed and then printed. Since the toner electric adsorption takes place on the surface of the photo conductor subjected to beam exposure with a constant light intensity or more, a change in the light output power of the semiconductor laser enables a change of the size (printing dot size) of a dot to be printed, thus realizing multiple density printing.
The inventors of the present application have analyzed and studied an exposing optical system for the purpose of obtaining high-resolution multiple density printing, and have found that, with respect to the size of a printing dot formed by a beam spot focused on the photo-conductor drum of a laser printer, it is difficult to obtain accurate levels of multiple densities, i.e., high-resolution multiple density printing in an area having small printing dot sizes.
That is, the prior art laser printer is arranged so that the light beam to be focused on the photo-conductor drum is obtained by collimating or converging a laser beam emitted from the semiconductor laser element with use of the aforementioned optical system and by changing the optical path of the beam for laser printing. Accordingly the light intensity of the light beam irradiated onto the photo-conductor drum is the light output power itself of the semiconductor laser (semiconductor laser element), which largely depends on the characteristic of the semiconductor laser element.
In general, with regard to the light output power of a semiconductor laser element, it is already known that the linearity of a light output power versus current characteristic is deteriorated in its low optical power range, but it is not recognized that the fact adversely affects high-resolution multiple density printing of the laser printer.
In other words, in the laser printer, as the number of power levels increases, the range of light output power of a laser beam used is required to be broad and correspondingly a low optical power range is also required to be inevitably used.
In a high-resolution laser printer, it is demanded that printing be carried out with a printing dot finely changing arithmetically in size, but irregular change in the printing dot size in the low optical power range makes it difficult to obtain high-resolution laser printing.
FIG. 22(a) shows a graph of a light output power versus current characteristic of a semiconductor laser element, and FIG. 22(b) shows, in a model form, an example of a printing dot changing arithmetically in size. In FIG. 22(a), positions denoted by white and black small circles are current positions at which the printing dot is to be formed, and the current value of each position is arithmetically selected.
In FIG. 22(a), a characteristic line A denotes an actual characteristic and a characteristic line B is an ideal characteristic desirable for multiple density printing. On the characteristic line B as the ideal characteristic, there is a clear inflection point in a low optical power part. In a large current area subsequent to the inflection point, the characteristic line A is linear (exhibits a linearity). Thus when the current value is arithmetically changed in the area exhibiting the linearity, the size of the printing dot can be sequentially arithmetically changed, for example, from 1 to 7, though partially shown in FIG. 22(b).
However, as shown by the characteristic line A in FIG. 22(a), the output of the laser beam emitted from the semiconductor laser element is nonlinear in its area having small light output powers (low optical power range), that is, the characteristic line A is bent or inflected downwardly. Thus when the current value is arithmetically changed in an area including the above nonlinear part, the size of the printing dot can be arithmetically changed in a high-current-value area, but the dot size can be changed non-arithmetically in a low-current-value area. As a result, the change of the printing dot size formed in the low-current-value area become non-arithmetical, thus making it difficult to realize high-resolution laser printing.
More specifically, in such a simple proportional control method that a driving current is divided into n levels, from an output power of level 1 to an output power of level n as shown in FIG. 22(a), the light intensity cannot be controlled at equally-spaced levels and thus it is difficult to obtain high-resolution laser printing.
With respect to such non-linearity of the light output power, a problem takes place when the change range of the light output power becomes broad, even in the invention (disclosed in JP-A-9-74251) for improving a spot size control in an electric photograph system or even in the invention disclosed in JP-A-3-269456. That is, in the invention disclosed in JP-A-3-269456, use of a light output power range to provide a good-linearity light output power versus current characteristic obtained by setting its sensitivity to a low level means that an increase in the number of light output power levels increases the light intensity of the maximum light exposing level, which results in that it becomes difficult to realize it because the light output power of the semiconductor laser has a limit and it also involves other problems with the performance and life of the photo conductor of the laser printer.
Even in the invention disclosed in JP-A-9-74251, the invention is valid so long as the linearity of the light output power is used in its good area, but the invention is not designed to improve the linearity of the light output power, and has the same problems as in the invention of JP-A-3-269456 in that the lowest light exposing level in the multiple density printing must be relatively high.
It is therefore an object of the present invention to provide an electric photograph system wherein an electric potential image is formed to realize multiple density printing while printing dots having different sizes are formed on a photo-conductor drum by a light intensity modulation method, and wherein a means for correcting a light output power characteristic of a light beam at a focusing position on the photo-conductor drum is provided in a semiconductor laser itself or in a passage area of a laser beam emitted from a semiconductor laser so as to exhibit a linearity not only in a low optical power range but also in other optical power range.
Another object of the present invention is to provide an electric photograph system which can arithmetically regularly change the size of a printing dot to be formed on a photo-conductor drum by a light intensity modulation even in a low optical power range and can realize high-resolution multiple density printing.
Typical ones of embodiments of the invention disclosed in the present application are briefly summarized as follows.
(A) In accordance with a first aspect of the present invention, there is provided an electric photograph system wherein a light beam emitted from a semiconductor laser is scanningly focused on a surface of a photo-conductor drum controllably rotated by an exposing optical system, and a light output power of the semiconductor laser is controlled so that an electric potential image is formed while printing dots having different sizes are formed on the photo-conductor drum to realize multiple density printing, and wherein a saturable absorber (correction means) exhibiting a saturable absorbing characteristic is provided for correcting a light output power versus current- characteristic of the semiconductor laser in such a manner that a light output power characteristic of the light beam at a focusing position on the photo-conductor drum exhibits a linearity even in a low optical power range.
The saturable absorber is formed (a) in a light guiding structure of a semiconductor laser, (b) in an insulator layer below an electrode away from the light guiding structure (optical waveguide), (c) on a facet of the optical waveguide of the semiconductor laser, or (d) on an optical window facet of a package having the semiconductor laser built therein.
(a) When the saturable absorber is provided in the light guiding structure of the semiconductor laser, (1) the saturable absorber is formed in the form of a non current injection area provided in a part of the light guiding structure of the semiconductor laser element.
(2) The saturable absorber is formed by providing a low density current injected area which is a part of the optical waveguide of the semiconductor laser element and which is smaller in current injection density than the other areas. For example, the low density current injected area includes a plurality of non current injected areas and a plurality of current injected areas.
When the saturable absorber is provided in the light guiding structure of the semiconductor laser, the intensity of the saturable absorption is suitably high enough to cancel the contribution of spontaneous light emission to the laser beam in the vicinity of a threshold current.
(b) The saturable absorber is formed in the insulator layer below the electrode away from the light guiding structure (optical waveguide). That is, the semiconductor laser has a semiconductor layer of a first conductivity type provided with a strip part which defines the optical waveguide, an insulator layer formed on the semiconductor laser of the first conductivity type other than the stripe part, and an electrode formed on the insulator layer and stripe part and electrically connected directly to the semiconductor laser of the first conductivity type or indirectly thereto via a single or a plurality of other semiconductor lasers of the first conductivity type. The insulator layer is made of a silicon dioxide film provided on the semiconductor laser of the first conductivity type, an amorphous silicon film provided on the silicon dioxide film, and another silicon dioxide film formed on the amorphous silicon film. The amorphous silicon film forms a saturable absorber.
(c) When the saturable absorber is provided in the facet of the optical waveguide of the semiconductor laser as an example, a thin film layer having a absorption characteristic to the laser beam is provided in a reflection film formed on the facet of the semiconductor laser element (semiconductor laser chip).
(d) When the saturable absorber is provided on the optical window facet of the package having the semiconductor laser built therein, a thin film layer having an absorption characteristic to the laser beam is provided in an anti-reflection coated film provided on a light exit window of the semiconductor laser package.
Such a saturable absorber may be, in principle, provided at any position in the optical path. For the purpose of causing light absorption to usually take place with a sufficiently weak light intensity, however, it is preferable that the saturable absorber is located in the optical path, in particular, at a position having a high light beam density.
In the case of the above means (A), since the saturable absorber acts to always reduce a predetermined light output power, the characteristic line of the light output power versus current characteristic of the semiconductor laser is lowered downwards as a whole, which results in that the light output power characteristic of the laser beam at the focusing position on the photo-conductor drum can exhibit a linearity even in a low optical power range as in the other optical power ranges. Thus when the current value for formation of an printing dot is arithmetically changed, a printing dot arithmetically varying in its dot size can be generated with a high accuracy. Accordingly high-resolution laser printing can be attained.
The saturable absorbing characteristic as used in this specification exhibits a phenomenon which follows. In such a substance that electrons at a first level absorb energy of photons to transit to its second level for light absorption, in general, as the light intensity becomes sufficiently strong, the number of electrons at the first level decreases while the number of electrons at the second level increases, which results in that it becomes difficult for the light absorption to take place. In other words, such a phenomenon takes place that light absorption is saturated. The occurrence condition of such saturated light absorption is determined by the transition probability of the two levels, level densities and the relaxation time of excited electrons. An substance wherein the above values are suitable and the saturation of the light absorption takes place in the light intensity range of a semiconductor laser, is known as a saturable absorber, which exhibits a strong light absorption characteristic for light having intensities of a constant level or less but exhibits substantially no light absorption characteristic for light having intensity exceeding the constant level.
The above and other objects and novel features of the present invention will become clear as the following description of the invention advances as detailed with reference to preferred embodiments of the invention as shown in accompanying drawings.